According to Jerry D. Gibson, THE COMMUNICATIONS HANDBOOK, CRC PRESS, Boca Raton 1997, chapter 37, pages 513–528 the migration of photonics into switching is occurring in a variety of ways. This is happening because of the large variety of switching tasks to be performed in a modern communication network. The different switching tasks range from ensuring that major trunks have the ability to be switched from one route to another when a catastrophic accident destroys a route, to the real-time switching on a per call basis. A fiber optic communication network is operating at data rates from a few megabit per second up to 2.5 Gb/s and beyond. At the 2.5 Gb/s rate, several thousand digitized voice channels, each operating at 64 kb/s, can be transmitted along a single fiber using time-division multiplexing. In reconfigurable optical networks, optical cross-connect switches or switching fabrics, which are controlled by a network management system, are used to adapt the communication system dynamically to high capacity demands that vary in time.
In the event of a failure the entire multiplexed signal of a failed link, connecting a first and second switching station within a network, may automatically be switched to a protection fiber. The multiplexed signal may also be routed via an optical cross-connect switch of a third switching station in order to increase channel capacity or to reroute traffic of a broken link.
Optical cross-connect switches, which are described for example in U.S. Pat. No. 6,002,818, are therefore used to provide switchable cross-connects between optical fibers. One such cross-connect switch comprises piezoelectric lateral actuators for the optical fibers. The switching is accomplished therein by using a piezoelectric actuator which laterally translates the input fiber relative to the axis of a first collimation lens, thereby enabling the optical beam to be steered to a second collimation lens where another piezoelectric actuator has translated the end of an outgoing fiber into the location where it receives the optical beam. In this technique of steering optical fibers, steering units are required at the input fibers and output fibers. Besides being costly, the physical size of these beam-steering units affects the overall size of the optical crossbar. Another cross-connect switch comprises a beam-steering unit with rotating prisms to provide beam deflection from a transmitter side to a receiver side.
Optical communication initially was proposed to be based on the idea of switching optical signals from a selectable one of a plurality of incoming fibers to a selectable one of a plurality of outgoing fibers by orienting the end of the selected incoming fiber relative to a lens thereby providing a radiation beam of variable direction. An example therefore is disclosed in U.S. Pat. No. 4,512,036. Therein the free end of an optical fiber is moved by a piezoelectric bender element. That free end provides a beam of electromagnetic radiation which either impinges on a freestanding lens or on a lens that is mounted at the free end of the bender element.
Optical steering units comprising rotating prisms or further optical elements are hence known which direct an optical signal, emanating from an incoming optical fiber, towards an outgoing optical fiber. Switching of the optical signals in such devices is therefore performed in free-space.
An optical cross-connect switch designed to switch optical beams in free-space is described in WO 00/79311. This optical switch comprises, mounted on a base, at least one controllable actuator for positioning an optical element to guide an optical signal on a selected free-space switching path, within an optical path plane parallel to the base, from an input optical fiber to an output optical fiber. The optical beam can be directed in free-space by means of diffractive, refractive or reflective optical elements. By using microelectromechanical systems (MEMS) for actuating the optical elements the optical cross-connect switch can be made in reduced size.
Micromechanical structures are the basis of microactuators that are developed for the application in several technical fields, particularly for optical systems. Microactuators operating based on thermal effects are described in J. Micromech. Microeng., 10 (2000) 260–264. Electrostatic microactuators are described in WO 99/37013. Piezoelectric microactuators are described in U.S. Pat. No. 4,512,036. Microactuators operating based on the generation of surface acoustic waves are described in JP 10-327590.
A further cross-connect switch using reflective optical elements for redirecting an optical beam is described in U.S. Pat. No. 6,144,781. In this system a reflective optical element is disposed at the intersection of a first and a second light beam emanating from a corresponding first and second input in such a way that the light beams are directed to a first or a second output respectively.
When placing diffractive, refractive or reflective optical elements in free-space within an optical cross-connect switch in order to redirect an optical beam, several aspects require consideration.
First of all the production, mounting and alignment of the optical elements involve considerable costs. Production of optoelectronic integrated circuits (OEICs) incorporating the optical elements creates further problems since the relatively large size of optical elements further requires a corresponding size of the free-space switching region resulting in a comparably large dimension of the optical cross-connect switch. Transferring an optical beam through several optical elements may in addition cause transmission losses.
It would therefore be desirable to improve the described optical cross-connect switches.
It would be desirable in particular to provide an optical cross-connect switch that can be produced at reduced cost.
More particularly it would be desirable to create an optical cross-connect switch that can be realized as an optoelectronic integrated circuit, using a process of comparably low complexity.
It would further be desirable to create an optical cross-connect switch incorporating a reduced number of externally produced optical elements in order to further reduce costs and to avoid transmission losses.
It would be desirable in particular to provide an optical cross-connect switch of smaller size thereby allowing the connection of an increased number of optical fibers for switching purposes.